1. Field of the Invention
The present invention relates to a method of positioning a movable body that is moved relative to a fixed body.
2. Description of the Related Art
In machines of various kinds, provided is a movable part that is moved relative to a fixed part. The accuracy in positioning a movable part relative to a fixed part affects largely the precision of the machine. For example, in an indexing mechanism or the like provided to an attachment of a machine tool, a poor positioning accuracy leads to an indexing error. As a result, not only the machining precision is deteriorated, but also the operation may possibly be delayed due, for example, to the need to perform again the indexing.
An attachment provided with an indexing mechanism is mounted on, for example, a ram spindle of a machine tool. The indexing is performed by using a motor linked to the ram spindle. The attachment provided with the indexing mechanism includes a fixed part and a swivel part. The fixed part is fixed to the machine tool while the swivel part is rotatably supported inside the fixed part. The swivel part is rotationally indexed by the ram spindle. A gear coupling is provided between the fixed part and the swivel part for clamping these two parts to each other. In accordance with an indexing instruction sent from a control unit of the machine tool, the ram spindle is rotated, so that the swivel part, which is connected to the ram spindle, is rotationally indexed. However, the indexing is sometimes not accurately performed due to disturbances including: the sliding resistance between the swivel part and the fixed part; a residual pressure in the hydraulic pressure circuit which is provided to the attachment; and the like. When the clamping operation is performed in a state where the indexing has not been accurately performed, the teeth in the gear coupling are not properly engaged. As a result, an index error occurs. An example of such an index error is that the teeth of a half of the gear coupling hit the tops of the teeth of the other half thereof. In such a case, the indexing operation has to be performed again. However, this leads to an operation delay and the like, and is thus not preferable in the operation using the machine tool.
In order to solve such defects, proposed is a method in which a feed back is obtained by externally providing a position detecting device. However, a position detecting device is not easily attached to a ram spindle or an attachment in view of a machining space. Such a position detecting device that can be provided while securing a machining space is complicated in structure and also expensive. In this respect, it has been proposed to provide a sensor to a gear coupling of an indexing device (for example, Japanese Unexamined Patent Application Publication No. Hei 6-182655).
However, the technique described in Japanese Unexamined Patent Application Publication No. Hei 6-182655 has the following problem. Specifically, according to this technique, it cannot be detected that the two halves of a gear coupling are not set in proper positions, unless the two halves come close to each other to some extent, that is, unless a clamping operation is started. However, when it is detected that the two halves are set in improper positions after the starting of the clamping operation, the indexing operation has to be eventually performed again.
In the fields of not only machine tools but also any machinery, a precise positioning of a movable body is demanded in various scenes while it is difficult to provide a position detecting device for positioning a movable body. Accordingly, such a problem as described above often occurs also in cases other than that of the indexing in a machine tool.